1. Field of the Invention
The present invention is directed to an improved surface based stimulation device that optimizes the stimulus current delivered while minimizing its duration resulting in higher efficiency and minimum power consumption.
2. Description of Related Art
A nerve cell can be excited externally by increasing the electrical charge within the nerve, thus increasing the membrane potential inside the nerve with respect to the surrounding extracellular fluid. U.S. patent application Ser. No. 11/146,522, filed on Jun. 7, 2005 and assigned to the same assignee as the present application, discloses an external transdermal nerve stimulation patch. This is but one example of a surface based stimulation device. The fundamental feature of the nervous system, i.e., its ability to generate and conduct electrical impulses, can take the form of action potentials (AP), which are defined as a single electrical impulse passing down an axon or fiber. This action potential (sometimes also referred to as a nerve impulse or spike) is an “all or nothing” phenomenon. Rheobase is the minimal electrical current of infinite duration (practically, a few hundred milliseconds) that results in an action potential. In addition, to a minimum intensity there is also a minimum amount of time necessary to excite the nerve. The minimum time referred to as Chronaxie is a duration of time that produces a response when the nerve is stimulated at twice the Rheobase strength. If either the stimulation time or stimulation intensity of the stimulation signal is not sufficient, the nerve will not fire an action potential. An exemplary strength-duration curve for excitable tissue shown in FIG. 1 denotes the Rheobase by reference element “1” and has a value of 0.25 V while the Chronaxie is represented by reference element “3” and has a value of approximately 0.23 ms.
When an external electrical stimulus is applied transcutaneously the complex impedance of the skin can alter the stimulus current. For example, if the capacitance of the skin increases, the amplitude of the stimulation signal may be sufficient to excite a nerve, however, an adequate amount of time may be lacking to fire the action potential. This is known as current decay. When a DC current is applied to the surface of the skin, a decay in current is observed until the net current is zero. Typically, this takes approximately 600 microseconds.
It is therefore desirable to develop an improved method and system that adjusts for changing impedance of the skin or decay in current.